No one saw it, not the millions watching on television, the thousands around the Florida launch site nor the astronauts watching the sky turn from blue to black as they thundered toward orbit.

The first glimmer of unspeakable disaster. A wisp of orange flame against the gleaming white steel booster rocket.

Films would later show it clearly. At 58.32 seconds into the flight, just after Challenger rammed through the sound barrier at about 24,000 feet, a small flame cutting like a welder's torch escaped the steel casing of its right-hand booster rocket and poured out into the near-vacuum on the edge of space.

It burned through on the side facing away from the Challenger and its enormous fuel tank and, aided by shock waves swirling around the base of the vehicle, erupted into an uncontrollable 5,900-degree torch licking around the bottom of the massive external fuel tank loaded with 500,000 gallons of liquid hydrogen and liquid oxygen. Neither Mission Control in Houston nor Dick Scobee's crew had a hint that anything was amiss.

In earlier days, a shuttle commander might have known that disaster loomed. But confident of the shuttle's reliability, NASA had removed much of the instrumentation that once relayed data on the performance of the solid boosters and the condition of the fuel tank.

But even as the assurances were exchanged between Challenger and Mission Control that the shuttle was operating at full throttle, the flame was spreading. Unbeknownst to Scobee or to controllers, the fire spurting through the booster wall caused a 4% drop in its thrust. Sensing the suddenly unbalanced flow of power beneath it, Challenger's computer swiveled the engine nozzle of the left booster and the three main engines of the spacecraft to the right, keeping the stricken vehicle flying true.

The orange flame, growing larger all the time, cooked the skin of the fuel tank as it spread precariously close to a 17-inch oxygen line linking the tank to the orbiter's engines. Reaching from the top of the tank to Challenger's engine compartment, the flame also spread upward, flowing around the spaceship as it throttled back to full power.

Less than 14 seconds after flame escaped the booster, the great aluminum tank fueling the orbiter's main engines was fatally damaged, with vapor streaming out of both the hydrogen and oxygen containers. Halfway up the tank's side the flames grew into a small fireball, and beneath Challenger's flat belly, not far from the crew compartment, there was an explosion sending yellow-orange flames over the sides of the spacecraft, expanding until Challenger was almost engulfed. Then in a flash, beginning with a shimmering halo at the top, the entire tank, still carrying tons of fuel, blew up in an enormous orange, blue and white shrapnel-filled fireball reminiscent of the burst of a nuclear weapon.

A day postponed for 25 years had come.

At an altitude of 50,000 feet, the orbiter the size of a DC-9 airliner was obliterated by the explosion, blown into countless pieces--its wings and tail ripped away, its fuselage apparently crushed like an egg, the fuel in its heavily loaded orbital maneuvering system adding to the explosion of the external tank.

Some who viewed slow-motion film from space agency cameras thought they saw the crew compartment thrown clear only to be destroyed by a secondary blast. The smaller flammable parts were consumed by the poisonous 10-mile-high funeral pyre. The insulating tiles of its skin, pieces of its skeleton, and the heavy structures of its engines and landing gear rained down on the Atlantic for an hour, according to the Air Force.

Scobee's crew members, pinned to their seats by the acceleration and exhilarated by the thrust of the five mighty engines beneath them, had no warning. Said former Apollo astronaut Tom Stafford: "They never knew what hit them."

Out of the center of the blast, the two solid boosters emerged intact and flew crazily upward, their engine nozzles automatically centered for straight flight after they were torn from the orbiter. The swelling fireball and the boosters' contrails forming a snaky "Y" shape against the sky were the first evidence of the worst tragedy of the Space Age.

For a few fleeting moments, there was hope that Challenger itself would somehow fly out of the fireball and turn back for the Cape for an emergency landing, or, at least, an attempt to ditch in the ocean. At the launch site, a few reporters started to run for the landing strip several miles away but stopped, realizing Challenger was gone.

At the Mission Control Center in Houston, consoles froze. On each screen, an "S" for "static" formed, meaning no information was coming back from Mission 51L. As flight controllers stared at their consoles in stupefied horror, the range safety officer, fearful that the uncontrolled boosters might turn toward a populated area, sent a radio signal blowing them apart.